Diaryl carbonates are important starting materials for the production of aromatic polycarbonates, hereinafter sometimes referred to simply as polycarbonate. Polycarbonate is a widely used raw material in many different manufacturing sectors. Due to the high hardness as well as good transparency of the material, it can be applied in applications as diverse as automotive windows and optical lenses. The demand for polycarbonate is believed to increase largely within the next years, which will require the production of polycarbonate to be improved in terms of efficiency and environmental impact.
Polycarbonates can be manufactured by polymerisation of a diaryl carbonate with an aromatic dihydroxy compound. Said aromatic dihydroxy compound may be bisphenolacetone (BPA or bisphenol A), i.e. 2,2-bis(4-hydroxyphenyl)propane. The production of polycarbonate by the polymerisation of diaryl carbonate with an aromatic dihydroxy compound is known from U.S. Pat. No. 5,747,609. This document also describes the production of diaryl carbonate from dialkyl carbonate with an aromatic alcohol. In this reaction a transesterification of dialkyl carbonate takes place whereby overall the diaryl carbonate is produced and as by-product alkanol is obtained. The dialkyl carbonate can be produced from an alkylene carbonate and an alkanol. This reaction yields as useful products not only the dialkyl carbonate that can be used for the preparation of diaryl carbonate, but also a useful alkanediol. For this reaction one may use the alkanol that has been liberated in the transesterification of dialkyl carbonate and aromatic alcohol.
In general, the above-mentioned direct reaction of a dialkyl carbonate with an aromatic alcohol producing a diaryl carbonate is not favored thermodynamically. For example, the reaction of dimethyl carbonate and phenol has an equilibrium constant of Keq=10−7. This implies that large excesses of reagents are necessary to produce diaryl carbonate. As a consequence, large reactors are necessary which increases overall costs and therefore diminishes efficiency.
Another known process for producing a diaryl carbonate from an aromatic alcohol is one wherein the aromatic alcohol, such as phenol, is reacted with phosgene (COCl2) instead of a dialkyl carbonate. However, this process has the inherent drawbacks of employing the harmful phosgene and creating chloride containing waste streams. Therefore, the use of phosgene evidently has a negative environmental impact.
Solutions to the disfavored reaction equilibria in the reaction of dialkyl carbonate with aromatic alcohol have been suggested. These solutions still involved the use of aromatic alcohol but no longer comprised a direct reaction of said alcohol with the dialkyl carbonate. The aromatic alcohol was converted into an intermediate that had a more favorable reaction equilibrium with dialkyl carbonate than the aromatic alcohol as such.
For example, in U.S. Pat. No. 5,543,546, it is proposed that in a first step phenol (PhOH) is converted into phenyl propionate and methanol (MeOH), by reacting it with methyl propionate. In a second step, the intermediate phenyl propionate is reacted with dimethyl carbonate (DMC) into methyl propionate and the desired diphenyl carbonate (DPC). The methyl propionate formed in the second step is recycled to the first step. The reactions in said two steps and the net reaction of the integrated process wherein methyl propionate is recycled are shown below:2 PhOH+2 methyl propionate→2 phenyl propionate+2 MeOH  (1)2 phenyl propionate+DMC→2 methyl propionate+DPC  (2)2 PhOH+DMC→DPC+2 MeOH.  Net reaction
The disadvantage of such process involving the use of phenyl propionate as an intermediate is the necessity of having to recycle methyl propionate to the first step of the integrated process. This reduces overall efficiency. Further, and more importantly, since phenol is still used in said process the problem of an unfavorable reaction equilibrium is in fact just shifted to the first step wherein the phenol is reacted with methyl propionate. This implies that also in this first step large excesses of reagents are necessary, in this case to produce the intermediate phenyl propionate.
Another intermediate proposed in prior art (see e.g. U.S. Pat. No. 4,533,504) is one which is formed by reaction of phenol with ketene in a first step, which results in phenyl acetate. In a second step, this intermediate phenyl acetate is reacted with DMC into methyl acetate and the desired DPC. Finally, in a third step, said methyl acetate is converted into ketene and methanol. The ketene formed in the third step is recycled to the first step. The reactions in said three steps and the net reaction of the integrated process wherein ketene is recycled are shown below:2 PhOH+2 ketene→2 phenyl acetate  (1)2 phenyl acetate+DMC→2 methyl acetate+DPC  (2)2 methyl acetate→2 ketene+2 MeOH  (3)2 PhOH+DMC→DPC+2 MeOH.  Net reaction
Said process involving the recycle of ketene suffers from the same disadvantages as discussed above in respect of the process wherein methyl propionate is recycled to the first step. In addition, the former process suffers from the disadvantage that the first intermediate compound first has to be converted into a second intermediate compound which can then be converted into the ketene to be recycled.
Another process proposed in prior art (see e.g. U.S. Pat. No. 5,349,102) wherein phenyl acetate is used as an intermediate and which suffers from the same disadvantages as the above process wherein ketene is used, is one wherein in a first step said phenyl acetate and acetic acid are formed from phenol and acetic anhydride. Just like in the above process, in a second step phenyl acetate and DMC are converted into methyl acetate and DPC. However, in the third step, said methyl acetate is reacted with carbon monoxide resulting in acetic anhydride which is then recycled to the first step. The reactions in said three steps and the net reaction of the integrated process wherein acetic anhydride is recycled are shown below:2 PhOH+2 acetic anhydride→2 phenyl acetate+2 acetic acid  (1)2 phenyl acetate+DMC→2 methyl acetate+DPC  (2)2 methyl acetate+2 CO→2 acetic anhydride  (3)2 PhOH+DMC+2 CO→DPC+2 acetic acid.  Net reaction
In general, all of the above-mentioned integrated processes are disadvantageous in that several intermediates must be reycled in large amounts. In addition, certain by-products produced have such boiling point which makes it hard to separate the final product or valuable intermediate products.
For example, in a case where diphenyl carbonate (and methanol) is produced from dimethyl carbonate and phenol, always some anisole (methoxybenzene) is formed as a by-product. Anisole can be formed by reaction of the phenol with methanol and/or the dimethyl carbonate itself. Anisole has a boiling point which is close to the boiling point of the mixed carbonate methyl phenyl carbonate, which is formed as an intermediate. In a case where not all of the intermediate mixed carbonate has been converted into final diphenyl carbonate, it is highly desired to recover said mixed carbonate and to have it further reacted into final diphenyl carbonate. However, such recovery requires separation of the mixed carbonate from the anisole, e.g. by distillation, which is cumbersome because of a small difference in boiling points.
A further disadvantage associated with the use of an aromatic alcohol, such as phenol, in the preparation of diaryl carbonates, such as diphenyl carbonate, is that in a fully integrated process where said phenol has to be produced on the site itself, coproducts are made in addition to the desired phenol which may be undesired. The industrially most important process for preparing phenol is one wherein cumene (made by alkylation of benzene) is oxidised into cumene hydroperoxide which is then converted into phenol and acetone. If the acetone cannot be used on the site itself in any other process, it has to be sold.
In view of the above disadvantages associated with the use of an aromatic alcohol, such as phenol, in the preparation of diaryl carbonates, such as diphenyl carbonate, it would be highly desirable to be able to provide a process for the preparation of a diaryl carbonate which would not use an aromatic alcohol as a starting compound, not in the specific process step which results in the production of the desired diaryl carbonate and neither in any preceding process step.